1. Technical Field
The present invention relates to measurement of degradation of a secondary battery.
2. Background Art
In recent years, with the increase in significance of environmental problems, the use of lithium-ion secondary batteries that are used in electric vehicles (EV), hybrid electric vehicles (HEV) and the like to electricity storage systems for storing surplus power produced by photovoltaic (PV) power generation has been studied. Lithium-ion secondary batteries are also attracting attention because they are effective in solving an environmental problem with secondary batteries such as lead-acid batteries.
Secondary batteries, however, have limited lifetimes and degrade by being repeatedly charged and discharged, and their battery capacity recovery to full charge (recovered capacity) is reduced. As a method of measuring the degree of degradation of a secondary battery, a method is known in which a battery is fully charged and then discharged; the current and the voltage are measured during this process; and the battery capacity is calculated by an integrating method. However, it is rarely the case that a secondary battery used in an electricity storage system is fully discharged. Therefore, it is not easy to measure the battery capacity by this method.
Patent Literature 1 discloses a method of calculating total real capacity (recovered capacity) of a secondary battery by utilizing the fact that the capacity of the secondary battery relative to the full-charge capacity is uniquely determined from the open voltage. In the method disclosed in Patent Literature 1, the discharge current is integrated in a load-connected state and the open voltage is thereafter measured in an unloaded state. A relative capacity value is obtained from the open voltage, and a total real capacity (recovered capacity) of the secondary battery is calculated from the integrated discharge current and the relative capacity value obtained from the open voltage.